1. Field of the Invention
This invention relates generally to the electronic device testing applications, and more specifically to a new and useful system and method for testing electrical circuits using a photoelectrochemical effect. Electronic device testing also includes medical device electrodes, microelectrodes, and nanoelectrodes.
2. Prior Art
A device with electrical traces or other electrically conductive paths, such as a medical device with electrode sites for stimulation and/or recording, typically requires one or more manufacturing tests to verify electrical continuity or impedance equivalent within the traces. These tests detect defects, such as unwanted open-circuits (breaks), high resistance, or short-circuits, within the conductive paths. Electrical continuity or impedance equivalent measurement in any passive electrical device requires contacting electrical pads. Depending on intent, a contact pad is variously known as a bond pad, terminal, test pad, via, or electrode. Conventional electrical continuity tests generally require two physical contacts to create an anode and cathode. For example when testing a medical device, a first physical contact may be the electrode site on the medical device, and a second one is on a proximal portion (e.g., bond pad) of the medical device.
However, many devices have relatively small dimensions that make physical contact with specific electrode sites or contact pads difficult and potentially damaging. In particular, medical and chemical sensors with electrode sites having diameters below approximately 100 μm risk damage as a result of physical contact with the sites during electrical continuity tests. Some microelectrodes, for example those that are typically used for sensing can be as small as 5 μm in diameter. Conventional electrical probe equipment (e.g., wire probes, MEMS probes, vertical probes) are generally not small enough to provide a reliable means of testing without damaging the microelectrodes.
Another method for electrical testing of an electrode or microelectrode is to submerge the electrode in an electrolyte and conduct an impedance measurement. The electrolyte must be of sufficiently low resistance to allow current to flow through the solution and back to a counter electrode. This technique applies a signal from the measurement tool to the circuit and requires very sensitive electronics and low capacitance leads to improve its dynamic range. Commercial impedance measurement devices also take two to twelve seconds for a single frequency measurement of 1,000 Hz (lower frequencies take longer). For commercial applications, that amount of time can be cost prohibitive.
Thus, there is a need in electronic testing field to create a new and useful system for testing electrical continuity or impedance equivalent between two physical contacts of a medical device, and the like. Moreover, the new test system and method must minimize risk to the structure of the physical contacts. Conventional neuromodulation devices have increasing electrode counts. In fact, thin-film MEMS-based devices may have hundreds of electrodes. A single wafer may contain 20,000 to 100,000 microelectrodes.
What is, therefore, needed is a reliable system for automated testing of thousands of devices on a wafer. The new test system must be especially sensitive to defects like opens and short-circuits. There is also a need to test small electrical contacts in integrated circuits without the expense of wire cantilever or MEMS-based cantilevers, commonly called “probes” in the electronics industry. MEMS cantilever devices are currently capable of probing 45-micron square bond pads with a minimum scrub length (sliding contact distance) of 15 microns. In that respect, the test system of the present invention has been used with sub-micron diameter contacts and been shown to provide relatively large amplitude signals. Such signals are reliably useful as a novel alternative to current testing methods.